Thin glomerular basement membrane disease (TBMD) is a hereditary nephropathy characterized by thinning of the glomerular basement membrane evinced by electron microscopy and, clinically, by isolated hematuria without extrarenal manifestations. Familial aggregation is found in 50-60% of cases, with autosomal dominant transmission. TBMD is considered to belong to the type IV collagen spectrum of diseases, since heterozygous mutations of the COL4A3 or COL4A4 gene have been detected in more than 30% of patients. The disease is found in 1-2% of biopsies, but the prevalence in the general population may be higher. The differential diagnosis with Alport's syndrome may be difficult and requires accurate family investigations, immunohistochemical evaluation of type IV collagen alpha chains in renal tissue and, if appropriate, genetic studies. Progression towards chronic renal failure, although rare, has been reported in some patients, and may be related to the phenotypical variability of COL4A3/COL4A4 mutations, to a missed Alport syndrome, or to superimposed glomerular disease. Patients suffering from TBMD and affected relatives should be periodically examined for signs of disease progression and informed about the possibility of transmitting the autosomal recessive form of Alport's syndrome.